The CPU will, but you would have to carefully choose the CPU cooler (stock one is too noisy).
The GTX760 will not unless you choose a slim profile card.
*There are different cases that I recommend instead. I like the ones that allow a full graphics card to exhaust directly out the side of the case.
Also, isn't the Alienware X51 an entire computer not a case? If so, why would you buy that and then change parts?
Also, the Alienware X51 alreay has an i7-4790 option. It's also OVERPRICED compared to building your own which I highly recommend ( pcpartpicker ).